A variation of taste-aversion and taste-preference learning paradigms are integrated into an experimental procedure to assess the hedonistic effects of initial morphine exposure, chronic morphine exposure, withdrawal from chronic morphine exposure, and termination of morphine withdrawal. Four experiments are proposed which were designed to clarify the role of learning factors in drug cross-tolerance, withdrawal, addiction, and recovery from withdrawal. Using rats, taste aversions and preferences will be assessed by way of Pavlovian pairings of novel tastes with first exposure to morphine, morphine following chronic morphine exposure, or morphine which terminates a period of withdrawal. D-amphetamine will also be assessed regarding its hedonistic properties and its cross tolerance to morphine effects.